1. Technical Field
The invention relates to electrophotographic image forming devices, and in particular, a heat dissipation structure of an optical scanning device for exposure and scanning of a photoreceptor.
2. Related Art
Optical scanning devices, which are also called “print heads,” are used in electrophotographic image forming devices, such as printers and copiers, for exposure of photoreceptor surfaces. Concretely, optical scanning devices each include a light source, a deflector, and an optical system. The light source irradiates the deflector with a light beam modulated by image data. The deflector rotates a built-in polygon mirror with a motor, which is hereinafter referred to as a “polygon motor,” and reflects the modulated beam by facets of the polygon mirror in turn, thus changing a deflected angle of the modulated beam periodically. The optical system focuses the deflected beam on a uniformly charged area of the photoreceptor surface. Caused by change in deflected angle, the beam spot travels on the photoreceptor surface in a direction, and thus, the surface is linearly exposed. In the linearly exposed area, according to change in exposure amount, a charge distribution forms a pattern corresponding to the image data, i.e. a line of an electrostatic latent image. The photoreceptor covers the outer circumferential surface of a rotator, such as a drum and a belt, and rotates along with the rotator during the exposure. In synchronization with the rotation of the photoreceptor, the optical scanning device repeats exposure of a line. This results in a plurality of exposed lines lying in the rotating direction on the photoreceptor surface, and thus, the electrostatic latent image extends two-dimensionally.
In a typical optical scanning device, a housing contains the polygon mirror, the polygon motor, and the optical system, thereby protecting optical elements from dust in the external air. In addition, the same housing allows all the optical elements to be fixed thereon, and thus, maintains a high accuracy of positioning of the optical elements. On the other hand, the housing tends to prevent heat dissipated by the polygon motor and its driver circuit from escaping to the outside. If the housing is overheated and thus excessively distorted, relative displacement between the optical elements has a high risk of becoming too large. If one or more of the optical elements are overheated and thus excessively deformed, change of their optical characteristics has a high risk of becoming too large. In either case, the location of an exposed area of the photoreceptor surface or the amount of exposure is subject to excessive error, which prevents image quality from being improved. In order to avoid this, it is important that the housing allows effective heat release from its inside.
An existing technology for heat dissipation is known, which is a structure insulating from the housing a path of escaping heat from the polygon mirror, i.e. a duct. For example, a structure disclosed in JP 2004-012859 includes the duct extending directly below the bottom surface of the deflector, more precisely, directly below the outer surface of a portion of the body of the deflector, and the inner surface of the portion faces the polygon motor. Each structure disclosed in JP 2005-024894 and JP 2010-113329 includes the duct in which the deflector is suspended in the air. In any of the structures, the duct allows air flows to carry heat from the motor to the outside of the housing, thus preventing the heat from invading the inside of the housing. In other words, the duct thermally insulates the optical system from the deflector.